This invention relates to a chucking device for a dental handpiece.
Up to now, a dental handpiece fitted with a cutting tool or bur has been used extensively for dental treatment This dental handpiece is provided therein with a driving member operated by pneumatic pressure or electrical power, a water feed conduit and air supplying and discharging conduits. The driving member is adapted for rotating the cutting tool or bur carrying a grinding end. In dental treatment, the cutting tool having the grinding end of the desired shape or size is selected as a function of the position or size of the site under treatment and attached to the tool head in exchange for a previously used tool.
FIG. 5 shows a tool head 1 of a conventional dental handpiece. The tool head 1 is constituted by a head housing 3 for attachment of a dental tool, such as a cutting tool 23, and a head housing jacket 2. A rotational sleeve 15 rotated by a rotor 15a and adapted for rotating a collet 10 as later described is mounted at the center of the head housing 3. Upper and lower ball bearins 5, 6 are accommodated within an inner casing 4. A cartridge cover 8 for the inner casing 4 is fitted to the upper end of the inner casing 4. Dampers 7 and 9 are provided between the cartridge cover 8 and the ball bearing 5 and between the ball bearing 6 and the inner casing 4, respectively, for taking up the vibrations produced by rotation of the rotational sleeve 15.
The aforementioned collet 10 having a plurality of vertically extending slits 11 is slidably accommodated within the rotational sleeve 15 for receiving and securing the cutting tool 23. The outer periphery of the lower end of the collet 10 has a tapered surface 12 which is flared towards below, while the upper end of the collet 10 is threaded and engaged by a mating set screw 13 having a flange 14. A coil spring 18 is interposed between the flange 14 and the upper end of the rotational sleeve 15 for perpetually biasing the collet 10 upwards with respect to the rotational sleeve 15. A step 16 is formed on the inner periphery of the rotational sleeve 15 for perpetually engaging with a corresponding step at the lower end of the tapered surface 12. Thus, the tapered surface 12 is pressed by the sleeve 15 and retained at the step 16, the tapered portion of the collet 10 being contracted in diameter.
A coil spring retainer 19 having a central collet insertion opening 20 is threadedly engaged with an upper central opening of the head housing 3 and a pushbutton 21 for retention and releasing of the cutting tool 23 by a one-touch operation is provided in the upper end opening of the coil spring retainer 19 so as to be perpetually biased upwards by a coil spring 22.
With the above construction of the dental handpiece, when attaching the cutting tool 23 to the tool head 1, the upper end of the cutting tool 23 is pushed into the inside of the tubular collet 10 by way of a cutting tool insertion tube 17, while the pushbutton 21 is pushed by finger pressure. At this time, under the thrusting pressure of the cutting tool 23, the collet 10 is enlarged in diameter due to extension at the slits 11 to accommodate the cutting tool 23. When the finger pressure on the pushbutton 21 is released, since the collet 10 is biased upwards by the coil spring 18 by means of the flange 14 of the set screw 13, the tapered surface 12 of the collet 10 is compressed radially inwardly so that the collet 10 holds the cutting tool 23 in position under the radially compressive force exerted by the tapered surface 12.
For extracting the cutting tool 23 from the collet 10, the pushbutton 21 is pushed down by application of a finger pressure against the force of the coil spring 22. This causes the set screw 13 to be lowered against the action of the coil spring 18, while causing the collect 10 to be lowered with respect to the rotational sleeve 15 to release the radial pressure exerted by the tapered surface 12 on the tool 23 so as to release the holding of the cutting tool 23 by the tapered surface 12 to thus permit facilitated exchange of the cutting tool 23.
With the above described conventional dental hanpiece, when a force in the tool releasing direction, i.e. in the downward direction in FIG. 5, is applied to the cutting tool 23, the tool 23 is moved downwards with respect to the rotational sleeve 15, leading to entraining the collet 10 in such downward movement At this time, the tapered surface 12 is flared downwards along the direction opposite to the tool inserting direction, that is in the releasing direction of the tool 23, so that, when the collet 10 is moved in this manner with respect to the rotational sleeve 15, the radially compressive holding pressure applied by the tapered surface 12 of the collet 10 against the rotational sleeve 15 is diminished. This tendency becomes more pronounced as the amount of downward movement of the collet 10 is increased, so that the tool 23 tends to be extracted when a force in the releasing direction is applied to the tool 23.